This invention relates to level control devices for liquids and other conductive materials and, more particularly, to such devices utilizing electronic circuitry in the probe circuit.
Liquid level control systems, such as low-water cut-off systems, are known in the art as exemplified in U.S. Pat. No. 3,834,357. These systems monitor the fluid level in vessels such as hot water boilers, or the like, by utilizing the conductivity of the fluid within the vessel. That is, an insulated probe projects internally of the vessel such that an electrical path is completed from the conductive portion of the probe through the liquid and to the grounded walls of the vessel, when the fluid is in contact with the probe. This circuit, in turn, completes a series circuit comprising the secondary winding of an isolation transformer and a relay having at least one set of contacts for controlling an external circuit such as a low-water alarm. An isolation transformer is utilized in these systems for safety considerations. That is, the isolation transformer avoids the possibility of having an electrically "hot" housing or exposed surface of any of the associated apparatus. However, in order to maintain a desired minimum level of sensitivity to ensure reliable detection when using fluids having low electrical conductivity, the secondary winding voltage is typically the same as the primary winding, which may be, for example, 120 or 240 volts. Notwithstanding the isolation transformer, this relatively high secondary potential may pose a safety problem such as, for example, to a workman who inadvertently physically contacts the exposed conductor of the probe. Thus, in these prior art systems, system safety and system sensitivity are in competition with one another.
Low secondary voltage systems, including electronic systems, have been proposed. However, these systems utilize relatively sophisticated and expensive techniques such as, for example, D.C. amplifiers wherein the A.C. probe signal voltage is converted to D.C. for further processing by the electronic circuit, such as by means of A.C. to D.C. converter or bridges and the like. The A.C. to D.C. interface has been necessary because it is desirable to provide an A.C. voltage in the probe circuit so as to avoid contamination of the probe due to electrolysis as results from strictly D.C. operation. In any event, these prior art liquid level control systems have resorted to expensive and complex circuit design techniques to provide a usable system.
These and other disadvantages are overcome by the present invention wherein a liquid level control system is provided utilizing a relatively low voltage secondary potential, the advantages of A.C. operation, and simple and relatively inexpensive circuitry which also utilizes the A.C. voltage signal of the probe circuit.